Means your car wasn't ordered with the 6FL option. You can retrofit it, but it isn't as simple as adding a cable. You have to replace the plug in the center console with the one that also has a USB port and then run the cable from that plug to the MULF module in your trunk, which you probably don't have, so you'll have to add that module as well.

I don't seem to have a USB audio input. Is there an cord i can buy that's plug and play? This is for a 2008 BMW M3 E90 with Navigation. Thanks in advance

I guess you have to use Aux port for playing music.
BTW I don't think 2008 M3 with USB can play MP3 from USB port. The USB port controls the ipod/iphone but sound is playing from Aux port with Y cable.
here is more info...http://www.m3post.com/forums/showthread.php?t=758529

So absolutely no USB retrofit available? FYI i have a 6 disc CD changer i don't use.

Maybe time to consider CIC upgrade and included a USB port. There are reputable vendors like BPMSport (MIke) and BimmerTech (Patryk) who can help you out. Because you don't have a good base to start with it could be little more expensive and labour intensive given running wiring through the car. Good luck!